Migration: separate core command line options from others.
Migration: separate core command line options from others.

This allows the core options to be re-used by other tools.

Change-Id: Iea9c18bf4568280dbf2a93ba43656b266226bcf5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/160341
Reviewed-by: Mike Fairhurst <2002604e76601be549134505841fea93de80dc45@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk